The present embodiments relate to T1 mapping in magnetic resonance (MR) imaging.
Quantification of T1 relaxation characterizes myocardial tissue to assess both ischemic and non-ischemic cardiomyopathies. Prior to the administration of contrast, an elevated value of myocardial T1 is associated with edema, which may be related to the inflammatory response to myocardial injury. Following the administration of a T1-shortening contrast agent, a shortened T1 corresponding to increased contrast agent concentration is associated with fibrotic scar or diffuse fibrosis, which has a greater extracellular volume than normal. Quantification of this extracellular volume fraction based on measurement of pre- and post-contrast T1-maps may be used to detect diffuse fibrosis, which is difficult to detect based solely on late enhancement imaging.
T1 mapping of the heart is a challenge mainly due to cardiac and respiratory motion. The normal T1 value of the myocardium is approximately 950 ms at a field strength of 1.5T. This time is on the order of the cardiac cycle, so cardiac motion is a problem. A modified Look-Locker Inversion Recovery (MOLLI) sequence may be used for myocardial T1 mapping. The pulse sequence for measuring inversion recovery is ECG triggered and acquires data at end-diastole when the heart is reasonably stationary. Unlike the standard Look-Locker (LL) T1 mapping that samples the recovery curve with single shot images in constant intervals after an initial preparation or inversion pulse, the MOLLI sequence typically splits the sampling of recovery curve across multiple heart-beats. To obtain sufficient samples for accurate T1 estimation at the desired inversion times (TI), multiple inversions and corresponding TI samples are used. The samples are acquired at the same end-diastolic phase, such as acquiring 6 to 11 inversion recovery (IR) samples at different TI times during a breath-hold.
MOLLI is still subject to motion artifacts. The heart may alter position over time or multiple cycles, not just during a cycle. The location of the heart may be different for end-diastole of one cycle than for another. In one approach to dealing with such motion, motion-free synthetic images presenting similar contrast to original MOLLI data are estimated. These synthetic images are registered to original MOLLI images to overcome the difficulties of largely varying contrast. However, the synthesis is a time consuming process and prone to error due to a low contrast-to-noise ratio around the zero crossing point. Coupled with time consuming iteration for T1 mapping, an undesired delay may occur.